The term “AlGaN” is used to denote a member of the AlxGa1-xN (0≦x≦1) material family having an aluminium mole fraction x that is non-zero but is less than 1. The complete material family, including x=0 and x=1, will be referred to herein as “(Al,Ga)N” for convenience.
Similarly, the term “InGaN” is used to denote a member of the InyGa1-yN (0≦y≦1) material family having an aluminium mole fraction y that is non-zero but is less than 1. The complete material family, including y=0 and y=1, will be referred to herein as “(In,Ga)N” for convenience.
The epitaxial growth of Group III nitride semiconductor materials on a substrate can be effected by molecular beam epitaxy (MBE) or by chemical vapour deposition (CVD) which is sometimes known as Vapour Phase Epitaxy (VPE).
CVD/VPE takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
In contrast to CVD/VPE, MBE is carried out in a high vacuum environment. In the case of MBE as applied to the (Al,Ga)N system, an ultra-high vacuum (UHV) environment, typically around 1×10−3 Pa, is used. A nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and species providing gallium and/or aluminium, and if desired also a suitable dopant species, are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The nitrogen precursor and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
At present, the majority of growth of high quality nitride semiconductor layers is carried out using the metal-organic vapour phase epitaxy (MOVPE) process. The MOVPE process allows growth to occur at a V/III ratio well in excess of 1000:1. The V/III ratio is the molar ratio of the group V element to the Group III element during the growth process. A high V/III ratio is preferable, since this allows a higher substrate temperature to be used which in turn leads to a higher quality semiconductor layer.
At present, growing high quality nitride semiconductor layers by MBE is more difficult than growing such layers by MOVPE. The principal difficulty is in supplying sufficient nitrogen during the growth process, and it is difficult to obtain a V/III ratio of 10:1 or greater during MBE growth of a nitride semiconductor layer. The two commonly used sources of nitrogen in the MBE growth of nitride layers are plasma excited molecular nitrogen or ammonia.
Many electronic or optoelectronic devices such as, for example, light-emitting devices such as light-emitting diodes (LEDs) or laser diodes incorporate (Al,Ga)N layers or (Al,Ga)N layer structures (heterostructures). The growth of (Al,Ga)N layers is therefore of considerable commercial importance.